Read the documentation on rehearsal marks. One of the default styles is
to use the barnumber.
\set Score.markFormatter = #format-mark-barnumbers
Incidentally, this will also probably get round the problem of keeping
track - I guess the problem is the rehearsal mark counter isn't
automatically updated in Shamus' workaround. Of course, if you're using
barnumbers as rehearsal marks you don't need to keep the counter
updated.

Hi Anthony,

You probably didn't read the whole thread (or the subject line:) ).
I am using format-mark-barnumbers. I just want to put a segno under
one and a coda sign under another.

I apologize in that I might not have been the least bit clear that I
wanted to use one of the automatically generated ehearsal marks.